# What would make a blue sky bluer?July 17, 2008 11:26 PM   Subscribe

What circumstances would make the sky bluer than it is normally?

I'm creating a world for a science fiction game that is generally Earth like, but its sky is bluer than the skies of earth. What would make this possible? A larger/smaller sun? Hotter/cooler sun? Different orbit? Different axial tilt?

As you can see, I know nothing about the matter. Fortunately, my scenario doesn't have to stand up to the scrutiny of editors and discerning readers--it's a game setting for a group of 7 of us--but I'd like it to have some basis in reality.
posted by happyturtle to Science & Nature (23 answers total) 3 users marked this as a favorite

A thinner atmosphere. The sky is bluer above you than towards the horizon because the scattered sunlight travels through less air. Viewed from on top of a high mountain, the sky is a deeper blue.
posted by weapons-grade pandemonium at 11:44 PM on July 17, 2008

Also, your eye's auto-white balance. If everything in your world is yellowish, most people will adjust their "white-balance" to perceive yellowish stuff as neutral and blue stuff as "bluer".
posted by qvantamon at 12:03 AM on July 18, 2008

The blue colour of the sky is due* to Rayleigh scattering, which occurs when the object doing the scattering is much smaller than the wavelength of the light being scattered.

The intensity of the scattered light is proportional to cos2θ where θ is the scattering angle and inversely proportional to λ4 where λ is the wavelength of the light being scattered. The shorter (bluer) wavelengths are therefore scattered (much) more than the longer green and red wavelengths. The Sun looks yellow because these green and red wavelengths are what's left over when you look at the Sun; above the atmosphere the Sun appears white.

A thinner atmosphere would make the sky bluer, but if you're looking for a more drastic change I would go with changing the tilt of the planet. The sky looks red in the evening because the light from the Sun is coming in at such a steep angle that the incoming blue and green light is scattered away from Earth, leaving only the red.

You could also change the colour of the sky by changing the Sun. As stars get cooler they get redder (Wein's displacement law), if there was no blue light to scatter then the sky couldn't look blue!

* ... mainly ...
posted by alby at 12:48 AM on July 18, 2008 [3 favorites]

A thinner atmosphere.

A cleaner, cooler ground atmosphere. The summer skies over London, Amsterdam, Paris, Rome, Atlanta, or Madrid are never, ever as blue as the summer skies over Stockholm.
posted by three blind mice at 12:59 AM on July 18, 2008

Hotter sun, less water vapour (clouds) and no particulate pollution in the sky therefore no Mie scattering (see alby's link).

Also, THICKER atmosphere. More atmosphere means that there's more Rayleigh scattering, which means that even during the middle of the day you'd have lighting like we do at sunset: yellowish direct sunlight (all the blue is scattered out of it) and deep, bright blue in the rest of the sky (where all the blue light went).

Get the atmosphere deep enough, you get purple sky.
posted by polyglot at 1:01 AM on July 18, 2008

Would a shift in atmospheric composition have an impact?

A different solar spectrum seems likely to have an impact, i.e. more photons in the blue wavelengths. Does anyone want to comment on what a sun might be like that would contribute to this?
posted by biffa at 2:26 AM on July 18, 2008

If you wanted to have a star with more "blue wavelengths" then you would just need a hotter star. The peak of the blackbody radiation curve shifts towards shorter and shorter wavelengths as temperature increases - that's why our cool star sends out lots of infrared and plenty of visible light, whereas really hot objects like black holes* send out ultraviolet and X-rays.

* Obviously black holes don't send out ultraviolet or X-rays, but the material spiralling into one does.
posted by alby at 4:08 AM on July 18, 2008

Perhaps something exploding in orbit carrying a substance that circles around earth and has spread out, resulting in a bluer sky?
posted by perpetualstroll at 6:54 AM on July 18, 2008

i agree with polyglot - you want a thicker atmosphere (from higher gravity?).
as others have said, it should also be cleaner (less water vapour, volcanic ash, pollution) because these larger particles scatter more uniformly (ie make things white). and a hotter star would indeed be bluer (but that has other consequences too - check stellar evolution models).
some of the rayleigh scattering is from gas molecules rather than small particles (aerosols etc). so a different gas composition would change things. this work (table 2, you want high cross section, sigma) shoes that an atmosphere rich in CO (carbon dioxide), CO2 (carbon dioxide), CH4 (methane), N2O (nitrous oxide, laughing gas), or SF6 (sulphur hexafluoride - if your planet had a lot of this, you might have gas lakes. see for example, here) would help.
posted by not sure this is a good idea at 7:06 AM on July 18, 2008

There are competing effects in the thinner/thicker atmosphere question. Red light mostly scatters forward, while blue light scatters omnidirectionally. A sunset is red in the west because the sunlight travels through more air; the blue gets scattered to the people to your west and red is all that's left. I think a thicker atmosphere would be reddish at midday, perhaps east-at-sunset blue-violet at the horizon. Under the ocean it is blue, blue all round with only a hint of the actual direction of the sun.

For composition effects consider the difference between Earth (blue sky) and Mars (red sky).

A fun idea, since this is for a story, would be a volatile ozone-like layer in the stratosphere that fluoresces in the blue. This would let you play with fun things. You could have "electric" colors. If the fluorescence was long-lived, the sky would glow for part or all of the night. If part of the night, you could learn things about the weather in the upper atmosphere from the brightness of the sky.

The bleach used to make super-white cloths and papers is actually a little fluorescent, but it's some complicated organic molecule. Of course there are bacteria that poop fluorescent dyes. Getting fluorescent bacteria in large quantities into the upper atmosphere might take some creativity.
posted by fantabulous timewaster at 7:39 AM on July 18, 2008

tangentially, when I wear my polarized sunglasses, the sky looks bluer then when I don't wear them.
posted by Wild_Eep at 7:57 AM on July 18, 2008

You could also have environmental factors which change the perception of the viewer, via brain or eyeball.
posted by biffa at 9:02 AM on July 18, 2008

when I wear my polarized sunglasses, the sky looks bluer then when I don't wear them

This is an effect of the scattering, discovered by François Arago (of Arago's Spot fame) in the early 1800s.

The blue scattered blue light is less polarised that the other unscattered light, so viewing the sky through a polarising filter cuts out more red and green and less blue, making the sky look bluer.
posted by alby at 9:12 AM on July 18, 2008

The blue scattered blue light is less polarised that the other unscattered light

why would unscattered light be polarised at all? the sun is a thermal source. this makes no sense at all.
posted by not sure this is a good idea at 12:50 PM on July 20, 2008

If the light from the sun is unpolarized, and one polarization tends to scatter, the unscattered light is polarized by subtraction. The pattern of polarization across the sky is different for different colors. Apparently some insects use this for navigation (all my links just got eaten, bummer).
posted by fantabulous timewaster at 3:22 PM on July 20, 2008

it's scattering, not reflection or filtering. so you don't preferentially remove one polarization - instead you generate new polarized photons (from photons absorbed "at random", unless somehow the scatterers are aligned). so at face value that still doesn't seem right. but i do see now how it could be more complex, so there may be something there...
posted by not sure this is a good idea at 8:30 AM on July 21, 2008

n.s.t.i.a.g.i., reflection is a special case of scattering. The physics is roughly the same as in Brewster polarization.

Any process that takes unpolarized photons and sends those with a certain polarization one way has to send those with the other polarization some other way. This is because light polarization is related to angular momentum, and as far as anyone can tell, angular momentum is an exact symmetry of nature. Filters absorb one polarization (that is, turn it into heat). Scatterers don't have this freedom.

You can use the angular momentum of (circularly polarized) light to drive a pendulum.
posted by fantabulous timewaster at 10:02 AM on July 21, 2008

ok, what i said about reflection/scattering doesn't help me look smart, but here is the mental model i am using:

there's a beam of light, coming from the sun, heading in some direction. some "scatterer" gets in the way and is excited/reradiates the light. in the forward direction there's no change in polarisation, because these scatterers are arranged at random - there's only a net decrease in intensity. the polarisation is visible at right angles to the original beam (and because of the axial symmetry your argument about angular momentum is not a problem).

in the case of a mirror things are different due to coherence - only in the direction expected from geometrical optics do you get coherent emission from the scatterers. that is not the case here, however. so i guess i should have said that it is incoherent scattering - it's not a process that involves larger scale structure and so has no preferred orientation for polarisation in the direction of the beam (unlike reflection or filtering by a mirror or filter). that's why the passed through light is unpolarised.

that's the intuition that's making me question what you are saying. if it's a poor model for rayleigh scattering i apologise, but i think it makes some sense.
posted by not sure this is a good idea at 8:25 PM on July 21, 2008

(ie no net change in polarisation, assuming many scatterers)
posted by not sure this is a good idea at 8:27 PM on July 21, 2008

and this article suggests bird navigation does require special conditions (when my symmetry arguments do not hold).
posted by not sure this is a good idea at 8:32 PM on July 21, 2008

You know, you're right. The separation between the two polarizations is between the light that gets scattered up or down and the light that gets scattered left or right, but the forward-scattered light doesn't have any preferred polarization.

By the way, your video above of the aluminum foil boat, floating on the SF6 lake, is pretty rad.
posted by fantabulous timewaster at 9:26 AM on July 22, 2008

oh gosh! thank-you. honestly didn't expect that (felt i had made so many mistakes i must have made another...). well, that's cheered me up. i am going to lok at the video now (i have noscript running on the browser which must have blocked it).
posted by not sure this is a good idea at 5:03 PM on July 22, 2008

oh that video! :)
posted by not sure this is a good idea at 5:03 PM on July 22, 2008

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